On photosynthesis

This little bit of theory is more for your interest, and can easily be skipped,
and/or forgotten. I provide it, however, because of personal interest, and
because it provides the scientific rationale for the discussions under lighting, artificial light, etc.

Green plants have the ability to absorb the sun's energy and convert it into
a more useable form, specifically, to use this energy for the construction
of new plant tissues. Two types of chlorophyll absorb most of this energy,
much of it in the red and violet-blue parts of the spectrum. Other pigments, including the famed carotenoids, are absorb solar energy.

As illustrated, light energy is extracted from most of
the visible spectrum, though green light is absorbed relatively poorly. This
is reflected back into the environment, causing plants to appear green. It
also shows why both the red and violet-blue parts of the spectrum are important to plant growth. This is the balance referred to at the start of the
section on artificial light, the balance that plant growth lights especially attempt to reproduce, and the one we try to approximate using the various
combinations that have been suggested above.